4D imaging of ZnO-coated nanoporous Al2O3 aerogels by chemically sensitive ptychographic tomography: Implications for designer catalysts

Hao Yuan (1), Hui Yuan (2), Travis Casagrande (2), David Shapiro (3), Young-Sang Yu (3), Bjoern Enders (4,5), Jonathan R. I. Lee (6), Anthony van Buuren (6), Monika M. Biener (6), Stuart A. Gammon (6), Theodore F. Baumann (6), Adam P. Hitchcock (1)
ACS Applied Nano Materials, 4, Issue 1, January 2021: 621–632. DOI: 10.1021/acsanm.0c02924

Keywords

ptychography; X-ray spectromicroscopy; tomography; atomic layer deposition; aerogel

Abstract

The 3D chemical structure (4D spectromicroscopy) of nanoporous Al2O3 aerogels coated with ZnO by atomic layer deposition (ALD) was studied by multienergy scanning transmission X-ray microscopy. These materials are representative of a class of designer catalysts in which the nanoporous support is prepared separately from the active catalyst material, which is subsequently introduced by ALD, thereby allowing independent optimization of the morphology, chemistry, and spatial distribution of the support and catalyst. The samples studied were prepared by Ga ion and Xe plasma focused ion beam (FIB) milling as well as drop casting from water suspension. Zn L and Al K edge spectra of six samples with three different ZnO loadings were measured to investigate how loading and different sample preparation methods affect the 3D distribution of the ZnO and Al2O3. Scanning transmission X-ray microscopy (STXM) and ptychographic imaging at two energies each at the Zn L3 and Al K edge were measured. The ptychography data were analyzed by using the SHARP reconstruction code to generate quantitative 2D chemical maps of the Al2O3 and the ZnO. The STXM and ptychography maps were then measured at a sequence of tilt angles, covering up to 160° of rotation. The 3D structure of the ZnO and Al2O3 was derived from the tilt series data by tomographic reconstruction using a compressed sensing algorithm. A two-dimensional spatial resolution (half-period) of 6 nm, measured by Fourier ring correlation, and a 3D spatial resolution (half-period) of 9 nm, measured by Fourier shell correlation, were achieved when using the COSMIC beamline at the Advanced Light Source (ALS). The results show that for all of the ZnO loadings studied there is nonuniform coverage of the ZnO on the Al2O3 aerogel framework. In addition, we found that both FIB methods create sample artifacts, although the distortion was less with Xe plasma than Ga ion FIB.

How Our Software Was Used

Dragonfly was used for the alignment, the segmentation and the 3D reconstruction of ptychographic tomography data. Plus, our program's bone analysis plug-in tool was used to calculate volume porosity and the volume fractions of the Al2O3 and ZnO components in the central 1 μm3 volume unit for the ALD materials. Finally, Dragonfly was used for a quantitative analysis of the pore network and pore connectivity in ALD samples.

Author Affiliation

(1) Department Chemistry & Chemical Biology, McMaster University, Hamilton, ON L8S 4M1, Canada.
(2) Canadian Centre for Electron Microscopy, McMaster University, Hamilton, ON L8S 4M1, Canada.
(3) Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
(4) National Energy Research Scientific Computing Center, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
(5) Department of Physics, University of California, Berkeley, Berkeley, California 94720, United States.
(6) Lawrence Livermore National Laboratory, Livermore, California 94550, United States.